Magnetic suspensions are commonly employed where it is necessary or desirable to provide antifriction bearings for a rotating member such as a shaft. In a typical configuration, eight (8) poles, a first set of opposing pairs of poles to control the position of the rotating member about a first axis and a second set of opposing pairs of poles to control the position of the rotating member about a second axis orthogonal to the first axis, are provided on a stator. Each pole pair of the opposing pole pairs of the first and second sets of pole pairs is typically wrapped with a pair of control coils in electrical series relation. Excitation of the series coil pair of any pair of poles of either set of opposing pole pairs induces an additive magnetomotive force, that attracts the shaft, and a flux that traverses a magnetic circuit that is constituted by those poles, the included part of the rotor backiron and rotor and the poles/rotor gaps. To reverse the direction of force that acts on the rotor, the opposing serially connected coils of a set of pole pairs are excited. The magnitude of the force is proportional to the flux density squared, and directly proportional to the pole pair area.
To improve the speed of response of the control coils, and to linearize force as a function of flux density, it is known to additionally wrap a bias coil on each pole, in series with all of the other bias coils, and to electrically connect in series the control coils of the opposing pairs of poles of each set of opposing pole pairs. The bias coils are typically excited to provide a constant flux density across the gaps, usually at one half the saturation flux density of the pole magnetic material. The flux produced by the bias coils does not create a net force. The control coils, connected in series for each axis, are activated to provide a control flux that is added to the bias flux on one end and subtracted on the other end to produce a net force. Reversing the current reverses the direction of force.
In the typical prior art magnetic bearing embodiment, the eight (8) pole pieces wrapped with control coils, or control/bias coils, to provide four (4) North and four (4) South poles were able to controllably suspend a rotating shaft about mutually orthogonal axes, but the utility thereof in many applications was limited in respect of speed, size and power. The faster the rotating member is to turn, the more quickly the rotor will be cut by the alternating North and South poles of the sets of opposing pole pairs of the heretofore known magnetic bearings, and the greater the eddy current and hysteresis effects that will be induced in the rotor thereof. These eddy current and hysteresis effects produce heat and torque on the rotating member, thereby placing an upper bound on the rotor's rotational speed beyond which the heretofore known magnetic bearings became ineffective.
For a given force, the series coils for a pole pair must provide a magnetomotive force sufficient to overcome the high reluctance of two air gaps. A high ampere-turn condition is required which results in high power input. Or, increased volume for a lower power coil is traded off for pole volume which results in less force capability.
Also, the stator backiron radial dimension is equal to the thickness of a pole piece in order to reach magnetic saturation simultaneously in the pole piece and backiron. This limitation on backiron thickness further limits the volume available to coils.